Loss of vacuum may occur suddenly as a result of a mechanical effect in an accident. This happens infrequently and is not considered but rather the much more often occurring cases in which air, through very minute leaks, very gradually enters the evacuated insulation space where it cools off. The loss of vacuum goes hand in hand with a higher heat influx so that the stored liquified gas rapidly evaporates.
As soon as all the liquid gas has evaporated, the container as well as the air which has entered the insulation space heats up. Since the air cannot escape fast enough through the small leak, an overpressure is produced which damages or destroys the container.
In order to prevent this type of damage, relatively simply constructed safety valves have been installed on the outer container. These valves essentially consist of a cylindrical valve housing and an equally cylindrical valve plug. The sealing was achieved by a radial squeezing of an O-ring between the valve housing and the valve plug. These valves were, however, not satisfactory. A certain compressive force on the sealing faces is namely required to guarantee the seal. The response pressure of the valve is higher than this compressive force but should not be too high since otherwise the function as safety valve is impaired or lost. The given response pressure is, however, almost always exceeded since the squeezing of the O-ring produces excessive friction forces on the slide faces. The reasons are fabrication tolerances, different surface condition, environmental influences and, in particular, the high coefficient of adhesion. Cases of damage, therefore, occurred attributed to the failure of this valve to function. Either the valve did then not open at all or only at an unacceptably high pressure.
These problems occur especially when small containers, pipe lines and other small vacuum spaces have to be safeguarded. In large containers, for example, vertical tanks, the safety valves have such a large diameter that the outside air pressure already provides a sufficient compressive force for the sealing element.